Dose comparison of proton beam therapy with and without a multileaf collimator in the treatment of multiple hepatocellular carcinoma

Background and purpose Proton beam therapy (PBT) offers dosimetric advantages over conventional photon-based radiation therapy in treating hepatocellular carcinoma (HCC) because it more effectively spares normal liver tissue. This study investigated the potential benefits of incorporating a multileaf collimator (MLC) into PBT to further reduce the radiation exposure of normal liver tissues.

Materials and methods We compared eighteen PBT treatment plans with and without MLC implementation, focusing on normal liver dose metrics. Two normal-tissue complication probability (NTCP) models were used to evaluate treatment-related liver side effects. Additionally, we examined the correlation between the characteristics of multiple treatment targets and the volume of liver sparing at low doses to determine the criteria for MLC application.

Results The V1Gy(RBE) to the normal liver was reduced by a median of 2.4% (IQR: 0.8-6.0%) in the MLC-based plans. Implementing an MLC significantly reduced treatment-related liver side effects according to the results of the NTCP models. The maximum increase in the shallow-depth dose was 4.8%. The liver-sparing gain at ≤ 1 Gy (RBE) and the cumulative minimum distance between multiple HCC lesions were positively correlated (rs  = 0.95).

Conclusions The integration of MLC into PBT planning effectively reduced low-dose spread and increased normal liver sparing, potentially lowering the risk of radiation-induced liver side effects. However, the associated increase in the maximum dose at shallow depths warrants careful consideration during plan optimization. The spatial relationship between multiple lesions may serve as a useful criterion for selecting MLC use in clinical settings involving multiple targets.